Mathematics is not a deductive science - that's a cliché... What you do is trial and error, experimentation, guesswork.
Paul HalmosMany teachers are concerned about the amount of material they must cover in a course. One cynic suggested a formula: since, he said, students on the average remember only about 40% of what you tell them, the thing to do is to cram into each course 250% of what you hope will stick.
Paul Halmos